1. Field of the Invention
This invention relates to a method of manufacture for structural sandwich panels, laminates, and profile shapes from polymer matrix composite material, such as fiberglass, graphite epoxy, or other matrix and reinforcement materials, especially when a resultant composite material is intended to have a very high structural performance (such as a high strength-to-weight ratio or high interlaminar shear strength). This invention further relates to such fabrication methods which are suitable for adaptation to mechanization and automation, such as pultrusion, continuous resin transfer molding, heated belt continuous laminating presses, and like means of art. (As used herein with respect to the present invention, matrix composite material refers to the adherent material which bonds reinforcement particles together; composite material used without the word "matrix" preceding "composite material" denotes the combined matrix and reinforcement.)
2. Description of the Related Art
Previously, parts comprised of reinforcement and matrix molecularly bonded to a core (including plate, sandwich panel, and profile shapes) have utilized a constant pressure compaction process.
The typical constant pressure process entails laying reinforcement and core material (either by hand or through the use of a tape-laying machine, a filament winding machine, or other machine) onto a mold or mandrel and, if the reinforcement is not pre-impregnated with matrix precursor, wetting it with such precursor (either manually or under pressure in a resin transfer process), and applying pressure and heat or other curing energy (such as electron beams, microwave radiation, ultraviolet radiation, or infrared radiation).
The pressure is often provided by a press, a vacuum bag (sometimes augmented with elevated air pressure in an autoclave), or other mechanical means. Other means, however, do exist to apply a clamping pressure. (See, e.g., U.S. Pat. No. 4,560,603 of Jeffrey A. Giacomel, which is entitled "Composite Matrix with Oriented Whiskers" and which discloses the use of electromagnetic fields for this purpose and U.S. Pat. No. 4,021,288 of Donald F. Hannon, which is entitled "Attachment for Converting a Sheet Laminating Machine" and which discloses the utilization of mechanical rollers to provide the requisite pressure.) And electrostatic force as well as centrifugal force is known in the prior art to be useful for creating the desired pressure.
The function of pressure during the curing process is to provide compaction of the reinforcement. Compaction is desired in order to squeeze excess matrix from the product so that the highest possible ratio of the volume occupied by reinforcement (a strong material) to the sum of the volumes occupied by matrix (a weak material) and void is achieved. (Such ratio is termed the "packing ratio.") This, of course, produces the strongest resultant material as a product.
Considerable effort is expended by manufacturers of the highest quality composite parts to assure that high compaction levels are achieved. In may cases, for example, a vacuum bag is fitted to the part being manufactured and pumped down after each few layers of reinforcement are laid down. This process of incremental compaction while the part is being manufactured is called "debulking." Only when all the layers are in place is the final vacuum bag fitted, the part moved to the autoclave, and high pressure applied together with heat in order to cure the product or part.
Pulsed pressure has been utilized in the process of U.S. Pat. No. 5,268,055; but that pressure is employed only during the preparatory debulking stage, not during the stage which produces a cured product. There is no recognition in U.S. Pat. No. 5,268,055 that pulsed pressure can be utilized at much lower forces than constant pressure during the stage which produces a cured product.
The compaction processes used to date, moreover, require that the resins serve as a lubricant so that, as clamping pressure is applied, the fibers (of the reinforcement) may slide past one another and seek the tightest packing. Generally, tight packing ratios can not be achieved without very great clamping pressures as well as heat (to reduce viscosity) since the fibers (of the reinforcement) tend to hold their positions because of surface friction. Very high pressures, while effective in achieving high packing ratios, however, attain such high packing ratios by cross-sectional deformation of and, often, damage to the fibers (of the reinforcement) and to any inclusions. (Inclusions are materials other than the composite material which are either on one side of the composite material or between successive layers of composite material and which may also extend into the composite material. Examples of inclusions are core material and fittings.) Such damage necessarily reduces the strength and integrity of the product. Moreover, it is difficult and expensive to achieve high clamping pressures; expensive autoclaves often develop a pressure of more than 500 psi.
When the principles discussed above for the prior art have been applied to the design of a machine for the continuous manufacture of laminated material, two types of machines have emerged.
The first such type are designated as Goldsworthy-type machines, in recognition of the patents issued to William B. Goldsworthy, e.g, U.S. Pat Nos. 4,498,941; 4,402,778; 4,420,359; and 4,495,021. In U.S. Pat. No. 4,498,941 Goldsworthy teaches "[a] method for high speed continuous production of reinforced plastic sheets and reinforced plastic sheet laminate structures. . . . One or more layers of resin-impregnated, fiber-containing, reinforced plastic composite material is brought into contact with surfaces of one or more endwise abutted relatively flat panels. The panel or panels are then passed between a pair of continuously rotating belts. A pressurized air body . . . forces the belts into intimate contact with the layers forming the reinforced plastic laminate structure. . . . The belts may be heated so as to facilitate the curing. . . ." This manufacturing machine, thus, employs continuous pressure and heat.
The second type of machine for the continuous manufacture of laminated material is pultrusion process machines in which dry material is pulled through a bath of resin and then through a die that performs the compaction by means of constant pressure.